1. Field of the Invention
This invention relates to a semiconductor device and, more particularly, to a semiconductor device whose conductive layer for wiring exhibits good corrosion resistance.
2. Brief Description of the Prior Art
In semiconductor devices, pure Al and Al base alloys have heretofore been generally used for wiring for such reasons (1) that the materials can easily provide ohmic contact with a semiconductor element, (2) that they have a high degree of electric conductivity, (3) that they can be easily evaporated and etched, and (4) that they are inexpensive. Among the Al base alloys, Al-Si alloy is a particularly known alloy which enhances the reliability of an ohmic joint portion, and Al-Cu alloy is a known alloy which prevents disconnection (failure) attributed to electro-migration and thus enhances the reliability of the Al wiring.
However, where a semiconductor device employing pure Al, an Al-Si alloy or an Al-Cu alloy as a wiring material is plastic-encapsulated, it is sometimes the case that moisture or water vapor reaches the semiconductor device by diffusion in the plastics or along the interface between a lead frame and the plastic, and that corrosion takes place in an electrode portion which is not formed with a protective film or in a wiring portion in which a protective film has a defect due to any cause, with the result that the wiring is disconnected.
In general, the following five factors are important in the environment in which the aluminum of the wiring conductive layer in a semiconductor device is corroded.
(1) Moisture is present. (2) Halogen ions such as Cl ions are present. (3) Dissolved oxygen is present. (4) The aluminum is connected to a more noble metal than aluminum. (5) Since the Al wiring on the semiconductor is a thin film having a width of several to several tens of microns and a thickness of several .mu.m or less, only a slight amount of corrosion will greatly influence the characteristics of the device unlike the case of the corrosion of usual bulky metals.
Prior-art semiconductor devices have been developed with little consideration given to the corrosive environment. It is, therefore, quite impossible to perfectly prevent the disconnection of the wiring conductive layer due to corrosion. Especially in the plastic-encapsulated semiconductor devices, defects due to corrosion of the wiring conductive layer frequently occur.
Corrosion of the wiring conductive layer of the semiconductor device presents aspects markedly different from those of the general corrosion on account of particularly the factors (4) and (5), so that the solution of the problem of corrosion has been difficult. More specifically, i) the geometries of the wiring conductive layer are very fine and especially the thickness is about 1 .mu.m in some cases, and even a slight amount of corrosion being merely a local corrosion in the closest proximity to the surface in the case of the bulky material becomes a very serious corrosion causing the disconnection in the case of the wiring conductive layer. ii) Since the wiring conductive layer has a voltage applied thereto or a different kind of metal connected therewith, the corrosion phenomenon differs from the general one. As regards ii), for example, where Al is immersed in aqua pura or in an aqueous extract of epoxy resin, the weight decrease due to corrosion is more conspicuous in the immersion in aqua pura as to the corrosion of the Al as it is, whereas the corrosion rate is ten or more times greater in the immersion in the aqueous extract of epoxy resin as to the corrosion of the Al under the state under which Au is held in contact therewith or a voltage is applied thereto.